1. Field of the Invention
The present invention relates to a semiconductor pressure sensor having a semiconductor pressure sensing element packaged in a resin package and also to a method of producing this semiconductor pressure sensor.
2. Description of the Related Art
FIG. 4 is a perspective view of a known semiconductor pressure sensor of the kind described. This semiconductor pressure sensor has a semiconductor pressure sensing element 30 (see FIG. 5) accommodated in a hollow resin package 10 which is composed of a bath-tub type resin base 12 and a resin cap 11 which is provided with a conduit 11a for transmitting the pressure to be sensed. External connection of the pressure sensing element 30 is achieved through leads 21 which extend outward from the package 10.
FIG. 5 is a perspective view of the known semiconductor pressure sensor illustrating the internal structure, when the semiconductor pressure sensor is being fabricated and the semiconductor sensor element 30 is carried by a lead frame, generally designated 20. The lead frame 20 includes the leads 21, a frame 22, a die pad portion 23 and suspension leads 24. The lead frame 20 is formed from a single sheet of metal by etching and press cutting such that the above-mentioned portions are formed integrally. Numeral 22a denotes a locating hole for locating the lead frame 20.
Each lead 21 has an inner lead portion electrically connected to the pressure sensing element 30, thus functioning as a lead for external electrical connection. The frame 22 is a connecting portion which surrounds the pressure sensing element 30. The frame 20 is used for the purpose of integrally holding the above-mentioned portions of the lead frame and is severed when it becomes unnecessary after the pressure sensor 30 has been correctly mounted in the package. The pressure sensing element 30 is die bonded to the die pad portion 23. After the lead frame 20 is cut in a flat form, the die pad portion 23 is displaced to such a level that the upper surface of the pressure sensing element 30 mounted on the die pad portion 23 becomes flush with the surface of the lead 21, e.g., by 2 to 3 mm. The suspension leads 24 are portions through which the die pad portion 23 is connected to the frame 22. The suspension leads 24 are deformed when the die pad portion 23 is displaced to maintain the depressed die pad portion 23 substantially parallel to the plane of the frame 22.
The semiconductor pressure sensing element 30 is die-bonded to the die pad portion 23 by means of a bonding agent 45 which may be a silicone resin. The semiconductor pressure sensing element 30 includes a glass base 32 and a semiconductor pressure sensor chip 31 bonded to the base 32. The thickness of the pressure sensor chip 31 is reduced at the central portion of the chip to, for example, several tens of .mu.m, such that a reference pressure chamber at vacuum pressure is formed between the glass base 32 and the pressure sensor chip 31. The pressure sensor chip 31 is bonded to the base 32 by an anodic bonding technique which is known. In operation, the central portion is deflected to a convex or concave shape in response to a change in the pressure applied to the sensor chip 31. Four gauge resistors 40 on the top of the sensor chip 31 surround the central portion of reduced thickness. These four gauge resistors 40 are electrically connected in a bridge circuit (not shown).
When the central portion of the chip 31 is deflected in response to a pressure change, the gauge resistors are respectively stressed and change their resistance values. Whether the resistance value of each gauge resistor 40 increases or decreases depends on the direction of the stress generated in the gauge resistor. As a result of the changes in the resistance values of the gauge resistors 40, the balance of the bridge circuit is lost so that voltages are generated between the opposing junctions of the bridge circuit. The voltages are picked up through bonding pads 41 which serve as electrodes. After the pressure sensing element 30 is die-bonded to the die pad portion 23, the bonding pads 41 on the pressure sensor chip 31 are connected to inner leads of the leads 21 by wire bonding through gold wires 42. Consequently, the voltages generated in the bridge circuit are delivered to the exterior of the pressure sensor through the leads 21. The pressure sensing element 30 bonded to the die pad portion 23 as shown in FIG. 5 is accommodated in a package 10. The pressure sensing element 30 is a known one and detailed description of the construction thereof is omitted because the construction does not form part of the present invention.
FIGS. 6 and 7 are sectional views of the known semiconductor pressure sensor in the fully assembled state. More specifically, FIG. 6 is a sectional view taken along the line 6--6 of FIG. 4, while FIG. 7 is a sectional view taken along the line 7--7 of FIG. 4. The pressure sensing element 30 die-bonded to the upper side of the die pad portion 23 is accommodated in a bath-tub type resin base 12. In order that the lower side of the die-pad portion is securely fixed to the inner bottom surface of the resin base 12, the pressure sensing element 30 is fitted in the resin base 12 after a resin adhesive 46 is applied to the inner bottom surface of the resin base 12. Consequently, the die pad portion 23 mounting the pressure sensing element 30 is fixed in the resin base 12 by means of the adhesive 46.
Then, a resin cap 11 is bonded to the resin base 12 by means of a bond or the like (not shown) such that the lead frame 20 is sandwiched between the resin cap 11 and the resin base 12. Practically, a thermosetting resin adhesive has been applied by printing to the joint surfaces of the resin cap 11 and the resin base 12 along their peripheries, so that the resin cap 11 and the resin base 12 are bonded together by thermally setting the resin adhesive. Thus, a portion of the lead frame 20 demarcated by broken line A is sandwiched between the resin base 12 and the resin cap 11. Subsequently, unnecessary portion of the lead frame 20 are severed and removed so that only the outer lead portions of the leads 21 are left to extend from both sides of the package 10. The outer lead portions of the leads are then folded downward as shown by broken line in FIG. 6 so that, when the package 10 is mounted on a circuit board (not shown), the outer ends of the outer lead portions of the leads 21 are connected to a circuit pattern on the circuit board. Although not shown, a protective silicone gel is supplied through the pipe 11a into the package 10 until the pressure sensing element and the gold wires are completely embedded.
The known semiconductor pressure sensor is produced by the process described hereinbefore. In order to prevent cutting and short-circuiting of the wires, the gold wires are preferably short. It is therefore preferred that the top surface of the semiconductor pressure sensing element where the bonding pads are provided be flush with the leads of the lead frame. Since the semiconductor pressure sensor element has a height of 2 to 3 mm, it is necessary that the die pad portion of the lead frame be displaced 2 to 3 mm below the plane of the frame, i.e., the plane of the surfaces of the leads. To this end, in the production of the known sensor, the lead frame formed from a single metal sheet by etching and stamping is formed in such a manner that the die pad portion is cantilevered by the suspension leads 24 and the suspension leads 24 are deformed by a press to displace the die pad portion. This known method, however, suffers from the following problem due to the large amount of displacement. Namely, after the die bonding of the pressure sensing element to the die pad portion and the wire bonding by the gold wires, the die pad portion carrying the pressure sensing element tends to be displaced vertically in the course of handling of the lead frame, e.g., fitting of the lead frame in the resin base, with the result that the gold wire loops are deformed or cut. In addition, since the die pad portion is cantilevered, it is necessary to fix the die pad portion in the resin base by means of an adhesive. Consequently, the pressure sensing element tends to be stressed by the resin base to which it is fixed, and is liable to be subjected to impact and vibration transmitted through the resin base.